Field of the Invention
In the acquisition of seismic data, seismic waves are used to interpret subsurface geological formations. In the late 1950s, Conoco, Inc., pioneered the development of a new type of geophysical prospecting technique, now generally known as “vibroseis” prospecting. Vibroseis prospecting employs a seismic vibroseis source used to generate a controlled wavetrain that propagates through the earth to be detected by seismic detectors. Vibroseis sources emit an energized wavefield over a selected time period known as sweep time. Seismic detectors detect earth movements during sweep time plus during an additional time period commonly referred to as listen time. Typically, the energization takes the form of a sinusoidal wavefield of continuously varying frequency applied to the earth or a body of water during a sweep time lasting from about two to about 20 seconds or even more.
Marine vibroseis seismic sources offer an alternative to marine airgun seismic sources. However, there are problems with marine vibroseis sources that must be solved before marine vibroseis can be accepted as a global production seismic source. Specifically, marine vibroseis sources must produce data comparable or superior to airgun-acquired seismic data. In addition, marine vibroseis must operate in an acquisition time frame that will equal or exceed airgun production. Historically, data quality comparisons of the two source methods tend to show comparable or better data in shallower sections of stacked data for vibroseis methods, while deeper stack data appears superior when acquired by contemporary airgun methods.
Whereas an airgun signal is impulsive and produces required energy almost instantaneously, production of marine seismic vibroseis energy requires a vibroseis projector emit energy over a specified time period. This presents both operational and geophysical problems for marine vibroseis methods. The time incurred emitting vibroseis energy in a contiguous full bandwidth manner compared to the impulsive airgun source is significant and, in the absence of simultaneous acquisition techniques, slows the rate of data acquisition.
In addition, and in the case of continuous acquisition, the time spent emitting vibroseis energy causes a smearing effect on subsurface horizons due to vessel motion. The amount of smearing is affected by the velocity of the vessel and the amount of time spent emitting vibroseis energy. In addition to sub-surface smearing, Doppler corrections applied to vibroseis data to correct for vessel motion become more significant the longer the time spent emitting vibroseis energy.
A problem common to both sub-sea surface source methods (impulsive and vibroseis) is the attenuation of very low-frequency and high-frequency bandwidth energy. This attenuation is caused by destructive interference due to sea surface reflection, which is affected by the proximity of the towed subsurface source to the surface.
Thus, it is of interest in the art to provide marine vibroseis methods and apparatus that operate in an acquisition time frame comparable to marine airgun seismic sources, while providing data comparable or superior to marine airgun seismic sources.